The compounds 4,4'-dihydroxybiphenyl and 4,4'-dicarboxybiphenyl are taught to be key monomers used as precursors for liquid crystal polymers and polyester film. 4,4'-dihydroxybiphenyl is currently prepared by a multi-step route and is not readily available for general use. 4,4'-diisopropylbiphenyl can be readily converted to either monomer by oxidation under the appropriate conditions. Accordingly, these monomers are prepared following equations 1 and 2. ##STR1##
It is generally accepted procedure to provide a numbering scheme for identifying the various isomers of biphenyl as follows: ##STR2## Obviously, the isomers derived from substitution in the 2, 3 and 4 positions are denoted as ortho, meta and para isomers, respectively.
The various combination of isomers of isopropyl alkylates of biphenyl are shown in Table I.
______________________________________ Isomers of Isopropyl Alkylates of Biphenyl Total Alkylate Number Isomer ______________________________________ Mono-IPBP 3 o, m, p Di-DIPBP 10 2,6; 2,4; 2,5; 3,5; 2,2'; 2,3'; 2,4'; 3,3'; 3,4'; 4,4' ______________________________________
As noted from the above table, the mono-alkylate of biphenyl has three possible isomers. These are derived from substitution of, in this case, propylene in the 2, 3 and 4 positions.
Dialkylation of biphenyl can occur in two ways, same ring and adjacent ring disubstitution. For same ring alkylation, it is likely that no ortho disubstitution can take place due to the stearic bulk of the isopropyl group. For same ring alkylation, the isomers expected are the 2,6; 2,4; 2,5; and 3,5. A greater number of isomers are possible for adjacent ring disubstitution, namely, 2,2'; 2,3'; 2,4'; 3,3'; 3,4;' and 4,4'. For the trialkylate, there are many more possible isomers making the total number of products possible in a typical alkylation of biphenyl with a non-shape selective catalyst quite large. If high conversion to diisopropylbiphenyl is sought, it is possible to create tetraalkylates complicating the mixture of products even further. As such, it is imperative for the practicality of any process for the production of 4,4'-diisopropylbiphenyl that the number of isomers be reduced so that the recycle and re-equilibration of byproducts be minimized.
The alkylation of biphenyl with olefins is a well known reaction. For example, K. Hyska teaches in Chem. Prun. 1971, 21, pp.264-70, that AlCl.sub.3 can be used as a catalyst for the propylation of biphenyl. However, the 3- and 4- isomers predominate for the case of the monoalkylate and complex mixtures of the dialkylate are present as reaction products. In addition, U.S. Pat. No. 4,480,142 discloses the use of NAFLON as a catalyst which gives an enhancement to the 2-alkyl products. This is probably due to the low reaction temperature of approximately 200.degree. C. taught by the referenced patent, as a result, the 2-isomer becomes the preferred product. As a result, the total amount of the 3,3', 3,4' and 4,4' isomers is low since the concentration of 3- and 4- isopropylbiphenyl is also low.
Notably, the selective preparation of 4,4'-diisopropylbiphenyl is taught in Japanese Kokai 63-122635. The Japanese reference teaches that the 4,4'-isomer can be made in relatively high selectivity using a mordenite zeolite catalyst while combining biphenyl with propylene at elevated pressures. The use of mordenite as the preferred catalyst was further substantiated by a recent report by Lee et al. (Catalysis Letters 2 (1989) 243-248). In this reference the author teaches the preference of a specific zeolite source and treatment by a dealumination procedure to obtain a suitable catalyst.
It was the object of the present invention to improve upon the selectivity which is taught in these references while avoiding the necessity for operating at either elevated pressures or treating the mordenite catalysts by costly and cumbersome dealumination procedures. It is particularly important to minimize the amount of propylene in contact with the catalyst at any given time. This is necessary in order to maximize the catalyst life since it is well known in the art that olefins can lead to the generation of carbonaceous residues on the surface of the zeolite catalyst thereby causing the catalyst to deactivate. As a result, the effective shape selective catalyst of this invention must be operated in such a manner as to minimize the propylene contact with the shape selective catalyst. This can be accomplished most effectively by operating at low propylene pressures. It is a further object of this invention to provide shape selective catalysts whose pore size and configuration are such that they maximize the yield of the desired 4,4'-diisopropylbiphenyl isomer relative to the sum of the other dialkylate species while minimizing the formation of higher substituted species. It has thus been determined that when the mordenite zeolite catalyst of these references is replaced by SAPO-11 or preferably ZSM-12 as the acidic crystalline molecular sieve of choice, the selectivity of 4,4'-diisopropylbiphenyl is enhanced while obviating the need for dealumination of the mordenite catalyst and operating the reaction at high propylene pressures.
It is sometimes found that beneficial performance, for example longer catalyst life and/or higher catalyst activity, ensues when hydrogen alone or in combination with hydrogen sulfide are added at moderate pressures to the reactants. The advantageous use of hydrogen in this way is often enhanced if a hydrogenating metal, for example nickel or palladium, is incorporated into the zeolite. Hydrogen sulfide is often used to partially deactivate the hydrogenating metal in such a manner as to only hydrogenate diene species present and not olefins or aromatics. A flow system can be used to operate the process of the invention, the unused hydrogen and/or hydrogen sulfide may be removed from the products and recycled.
The zeolite catalyst may be used in the form of suitable aggregates which are prepared by well known techniques, for example with or without a binder such as alumina, or by inclusion into a matrix such as silica-alumina, in a manner analogous to that often used in the manufacture of fluid catalytic cracking catalysts.
If desired, the external surface of the zeolite may be poisoned so as to reduce or remove catalytic activity thereon, especially the catalysis of unwanted reactions, for example the isomerization of the selectively alkylated biphenyl compounds of this invention. In some cases, zeolites treated with magnesium, phosphorous or silicon compounds have beneficial properties in the process of this invention.